1. Field of the Invention
The present invention relates to an image forming technique of the electrophotographic type for a printer, a copier machine, a facsimile machine and the like, and more particularly, to an image forming technique which uses liquid development as a development method.
2. Description of the Related Art
Such an image forming apparatus of the electrophotographic type has been commercialized in which an exposure unit exposes a photosensitive member (latent image carrier) which has been charged up, thereby forming an electrostatic latent image on the photosensitive member, a developing unit makes toner adhere to the photosensitive member, thereby visualizing the electrostatic latent image and forming a toner image, and this toner image is transferred to a transfer paper, thereby obtaining a predetermined image. Known as the development method for the developing unit is a liquid development method using a liquid developing agent (liquid developing agent) in which toner is dispersed in a carrier liquid. Known as an image forming apparatus using such a liquid development method is a structure which comprises: a developer housing portion holding a liquid developing agent; a coating roller which is immersed in the liquid developing agent, and accordingly carriers on its surface the liquid developing agent and scoops up the liquid developing agent; a developer roller (developing agent carrier) which abuts on the coating roller and gets coated on its surface with the liquid developing agent; and a photosensitive member which abuts on the developer roller so that the liquid developing agent develops an electrostatic latent image carried on the photosensitive member. In the image forming apparatus having this structure, as the developer roller is coated with the liquid developing agent which has been carried once on the surface of the coating roller, the surface of the developer roller is uniformly coated with the liquid developing agent. The electrostatic latent image carried on the surface of the latent image carrier is developed with the liquid developing agent thus uniformly applied to the developer roller, an image without a varying density is obtained.
However, in the image forming apparatus having this structure, the liquid developing agent, which has built up at the edge surfaces of the coating roller immersed in the liquid developing agent, may be blown up to the developer roller and the photosensitive member as the coating roller rotates. When the liquid developing agent gets blown up to the developer roller and the photosensitive member from the edge surfaces of the coating roller, the amount of the liquid developing agent existing in a nip portion between the developer roller and the photosensitive member becomes uneven all over the nip area. As a result, an image which is formed has an uneven density, which deteriorates the accuracy of development.
As a solution to this problem, the following techniques have been proposed. In the apparatus described in Japanese Unexamined Patent Publication No. 2000-235306, since a developer roller is longer along the width direction than a coating area of a coating roller but shorter than the length of the coating roller including a non-coating area, it is possible to prevent blowing up of a liquid developing agent which has built up at the edge surfaces of the coating roller. In short, as the overall length of the coating roller is longer than the length of the developer roller, the both edges of the coating roller will not abut on the developer roller, thus preventing the liquid developing agent which has built up at the both edge surfaces of the coating roller from getting blown up to the developer roller. Further, since the overall length of the developer roller is longer than a coating area length of the coating roller, the both edges of the developer roller will not abut on the coating area of the coating roller, thus preventing stay of the liquid developing agent at the both edge surfaces of the developer roller. In this manner, it is possible to prevent deterioration of an image quality such as an uneven density of an image attributable to blowing up of an unwanted liquid developing agent from a coating roller to a developer roller.